It's all in the intent.

A blunt truth is one that attempts at making a person see a valid point; saying "you are acting like a thug" or "you aren't going to attract better dates without taking showers" is truthful, and attacks behavior while attempting to get a person to see things differently. An insult is to say "you are a thug" or "you are an ugly mess that hot people don't want to date;" the intent is to devalue the person, thus making it a case of words as weapons instead of critical truths.